The lithographic printing plate generally consists of a lipophilic image part of receiving an ink in the printing process and a hydrophilic non-image part of receiving a fountain solution. The lithographic printing is a printing method where the attachment of ink to the surface of a lithographic printing plate is made to differ between the ink-receiving part assigned to the lipophilic image part of the lithographic printing plate and the fountain solution-receiving part (ink non-receiving part) assigned to the hydrophilic non-image part by utilizing the property of water and printing ink repelling each other and after inking only the image part, the ink is transferred to a printing material such as paper.
For producing such a lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (photosensitive layer, image recording layer) has been heretofore widely used. Usually, a lithographic printing plate is obtained by a plate-making method of exposing the lithographic printing plate precursor through an original image such as lith film and while allowing the image recording layer in the portion working out to an image part to remain, removing by dissolution the image recording layer in other unnecessary portions with an alkaline developing solution or an organic solvent to reveal the hydrophilic support surface, thereby forming a non-image part.
In this way, the plate-making process using a conventional lithographic printing plate precursor requires a step of removing by dissolution the image recording layer in the unnecessary portion with an alkaline developing solution or the like after exposure and in view of environmental protection and safety, the problem to be solved includes processing with a developing solution closer to the neutral region or less discharge of waste solutions. In particular, the treatment of waste solutions discharged along with the wet processing is recently a great concern to the entire industrial world from the standpoint of consideration for global environment, and the demand for solving the problems above is becoming stronger.
On the other hand, a digitization technique of electronically processing, storing and outputting image information by using a computer has been recently widespread and various new image-output systems coping with such a digitization technique have been put into practical use. Along with this, a computer-to-plate (CTP) technique is attracting attention, where digitized image information is carried on a highly converging radiant ray such as laser light and a lithographic printing plate precursor is scan-exposed by this light to directly produce a lithographic printing plate without intervention of a lith film. Accordingly, one of important technical problems to be solved is to obtain a lithographic printing plate precursor suitable for such a technique.
As described above, preparation of the developing solution to a lower alkalinity and simplification of the processing step have been increasingly desired more than ever in terms of both the concerns for global environment and the conformity with space saving and low running cost. However, the developing process generally comprises three steps of developing the plate with an aqueous alkali solution at a pH of 10 or more, washing out the alkali agent in a water washing bath, and treating the plate with a gum solution mainly comprising a hydrophilic resin. Therefore, the automatic developing machine itself occupies a large space, and additionally, problems in view of environmental protection and running cost remain to be solved, such as treatment of development waste, water-washing waste and gum waste.
To solve these problems, for example, JP-A-2002-91016 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes a development method using an alkali solution containing a nonionic surfactant having a pH of 10 to 12.5, but the photosensitive composition contains an alkali-soluble polymer and this leads to a problem that development cannot be performed at a pH lower than the pH specified above. Also, for example, Japanese Patent No. 2,938,397 (corresponding to EP0770494A2) describes a lithographic printing plate precursor where an image forming layer comprising a hydrophilic binder having dispersed therein a hydrophobic thermoplastic polymer particle is provided on a hydrophilic support. This lithographic printing plate precursor can be image-exposed using an infrared laser to cause thermal coalescence of hydrophobic thermoplastic polymer particles and thereby form an image, then mounted on the cylinder of a printing press, and on-press developed with a fountain solution and/or ink.
Such a method of forming an image by the simple thermal fusion and coalescence of fine particles has a problem that the image strength (adhesion to support) is very low and the press life is insufficient, despite good on-press developability.